I think when you use XXX like that the parser thinks it's an undefined constant and according to the PHP manual:

If you use an undefined constant, PHP assumes that you mean the name of the constant itself, just as if you called it as a string (CONSTANT vs "CONSTANT"). An error of level E_NOTICE will be issued when this happens.

If I understand the manual correctly E_NOTICE is not reported by the default setting in the php.ini.